In Patent Literature 1, the applicant of the present application suggested a gas-liquid mixing apparatus that is configured to mix, dissolve and segmentalize substances of a liquid phase, a gas phase and a solid phase for fluids such as water. According to this gas-liquid mixing apparatus, for example, in the case when water and oxygen are used as materials, oxygen water (water in which oxygen is dissolved) having a characteristic that a state under which an amount of dissolved oxygen of 25 ppm or more is maintained for 35 days or more even under an open atmosphere can be produced. Furthermore, for the amount of the dissolved oxygen in such oxygen water, for example, various devices for measuring dissolved oxygen utilizing known methods such as a diaphragm electrode method, a Winkler method and a fluorescence method are used.